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Air Pressure: The Chemistry of Weather Forecasts

This article was originally posted on RealClearScience.

Everybody in Seattle knows Jeff Renner. He’s the weather guy on KING5 News, the local NBC affiliate. Each night at around 11:15 p.m., he provides us with little nuggets of knowledge, from the extended forecast to the type of tires we should use when crossing the mountains.

Like all scientists, Mr. Renner has his own special lexicon. Meteorologists are known to use some phrases which may not make an awful lot of sense to the average person. For instance, you might very well hear your weather guy (or gal) say, “The barometic pressure is 30.2 inches and falling.” What on Earth does that mean? Well, it helps to know a little chemistry.

Barometric pressure — which is measured using a barometer — is another name for atmospheric (air) pressure. This is the pressure that air molecules exert on the surface of the Earth, largely because of their weight. But, pressure isn’t measured in inches; it is measured in pounds per square inch, millibars, pascals, or some other unit. So, why does your meteorologist measure pressure in inches?

The term is a throwback to the days when barometers were made using mercury. (Today, they are digital.) A glass tube was filled to the rim with mercury, and then it was inverted and placed into a reservoir that also contains mercury. The reservoir is open to the atmosphere. Air molecules that push down on the mercury in the reservoir cause mercury in the tube to rise. If atmospheric pressure increases, the mercury rises further; if atmospheric pressure decreases, the mercury level falls. (Check out this video on how to make a classical mercury barometer.)

As it turns out, “normal” atmospheric pressure at sea level causes mercury to rise to a level of 760 millimeters, written as 760 mmHg. Since Americans reject the metric system, we convert 760 mm to 29.92 inches. Atmospheric pressure typically fluctuates between 29 and 31 inches, with higher altitudes experiencing lower air pressure.

Now, we can translate that mystifying statement, “Barometric pressure is 30.2 inches and falling,” into useful English. What your weatherman meant was, “If we were using an old-school barometer, the mercury would be 30.2 inches high and falling.”

Note that the most important part of that statement isn’t the numerical value of the atmospheric pressure, but the fact that it is falling. Falling air pressure indicates that bad weather is coming, while rising air pressure indicates good weather.

Bizarrely enough, some people can sense changes in atmospheric pressure. You probably know an old person who says, “Storm’s comin’; I can feel it in my bones.” Old people aren’t making that up. People with arthritis are sensitive to low atmospheric pressure because their bones expand a little, causing pain.